msgget(2)

Synopsis

Description

The
msgget()
system call returns the System V message queue identifier associated
with the value of the
key
argument.
A new message queue is created if
key
has the value
IPC_PRIVATE
or
key
isn't
IPC_PRIVATE,
no message queue with the given key
key
exists, and
IPC_CREAT
is specified in
msgflg.

If
msgflg
specifies both
IPC_CREAT
and
IPC_EXCL
and a message queue already exists for
key,
then
msgget()
fails with
errno
set to
EEXIST.
(This is analogous to the effect of the combination
O_CREAT | O_EXCL
for
open(2).)

Upon creation, the least significant bits of the argument
msgflg
define the permissions of the message queue.
These permission bits have the same format and semantics
as the permissions specified for the
mode
argument of
open(2).
(The execute permissions are not used.)

If a new message queue is created,
then its associated data structure
msqid_ds
(see
msgctl(2))
is initialized as follows:

msg_perm.cuid
and
msg_perm.uid
are set to the effective user ID of the calling process.

msg_perm.cgid
and
msg_perm.gid
are set to the effective group ID of the calling process.

The least significant 9 bits of
msg_perm.mode
are set to the least significant 9 bits of
msgflg.

msg_qnum,
msg_lspid,
msg_lrpid,
msg_stime,
and
msg_rtime
are set to 0.

msg_ctime
is set to the current time.

msg_qbytes
is set to the system limit
MSGMNB.

If the message queue already exists the permissions are
verified, and a check is made to see if it is marked for
destruction.

Return Value

If successful, the return value will be the message queue identifier (a
nonnegative integer), otherwise -1
with
errno
indicating the error.

Errors

On failure,
errno
is set to one of the following values:

EACCES

A message queue exists for
key,
but the calling process does not have permission to access the queue,
and does not have the
CAP_IPC_OWNER
capability.

EEXIST

IPC_CREAT
and
IPC_EXCL
were specified in
msgflg,
but a message queue already exists for
key.

ENOENT

No message queue exists for
key
and
msgflg
did not specify
IPC_CREAT.

ENOMEM

A message queue has to be created but the system does not have enough
memory for the new data structure.

ENOSPC

A message queue has to be created but the system limit for the maximum
number of message queues
(MSGMNI)
would be exceeded.

Conforming To

SVr4, POSIX.1-2001.

Notes

The inclusion of
<sys/types.h>
and
<sys/ipc.h>
isn't required on Linux or by any version of POSIX.
However,
some old implementations required the inclusion of these header files,
and the SVID also documented their inclusion.
Applications intended to be portable to such old systems may need
to include these header files.

IPC_PRIVATE
isn't a flag field but a
key_t
type.
If this special value is used for
key,
the system call ignores everything but the least significant 9 bits of
msgflg
and creates a new message queue (on success).

The following is a system limit on message queue resources affecting a
msgget()
call:

MSGMNI

System-wide limit on the number of message queues.
Before Linux 3.19,
the default value for this limit was calculated using a formula
based on available system memory.
Since Linux 3.19, the default value is 32,000.
On Linux, this limit can be read and modified via
/proc/sys/kernel/msgmni.

Linux notes

Until version 2.3.20, Linux would return
EIDRM
for a
msgget()
on a message queue scheduled for deletion.

Bugs

The name choice
IPC_PRIVATE
was perhaps unfortunate,
IPC_NEW
would more clearly show its function.

See Also

Colophon

This page is part of release 3.80 of the Linux
man-pages
project.
A description of the project,
information about reporting bugs,
and the latest version of this page,
can be found at
http://www.kernel.org/doc/man-pages/.

License & Copyright

Copyright 1993 Giorgio Ciucci
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Added correction due to Nick Duffek , aeb, 960426
Modified Wed Nov 6 04:00:31 1996 by Eric S. Raymond
Modified, 8 Jan 2003, Michael Kerrisk,
Removed EIDRM from errors - that can't happen...
Modified, 27 May 2004, Michael Kerrisk
Added notes on capability requirements
Modified, 11 Nov 2004, Michael Kerrisk
Language and formatting clean-ups
Added notes on /proc files